1. Field of the Invention
The present invention relates to an X-ray imaging apparatus, and particularly relates to setting of a radiation field of X-rays.
2. Description of the Related Art
In X-ray diagnostic apparatuses, in order to protect a patient, i.e., an object to be inspected, from unwanted radiation exposure, X-ray protection standards (for example, see JIS Z4701) are established, for example, such that a deviation between the boundary of an X-ray radiation field and the boundary covered by an X-ray detector should not exceed three percent of SID (source image distance, hereinafter abbreviated as SID, a distance between the focus of an X-ray tube and the X-ray detector).
For this reason, X-ray diagnostic apparatuses include a function, such as an auto-collimation function, to avoid unwanted radiation exposure. The auto-collimation function is a function such that, as shown in FIG. 14, a radiation field is narrowed by closing blades of an X-ray beam limiting device 40 automatically when an X-ray detector 70 moves away from an X-ray tube 30.
The auto-collimation function can prevent unwanted X-rays outside a beam receiving area of the X-ray detector 70, however, the auto-collimation function cannot prevent X-rays outside a ROI (Region of Interest) in the radiation field (for example, an edge part of the radiation field). Therefore, to restrict radiation of X-rays outside the ROI, an operator needs to operate the X-ray beam limiting device 40 manually and to narrow an X-ray beam.
Therefore, X-rays may be irradiated to a position deviated from the target position of the patient P when tabletop rolling for rotating a tabletop 50 around an axis in the head-tail direction of a patient P is carried out as shown in FIG. 15 in order to facilitate the operator's approach to the patient, and this leads to a problem that the patient is exposed to unwanted radiation.